I haven't noticed any such activity. I often look at the post count from time to time and have never observed it decreasing. Could you provide an example (or multiple if possible) of posts that have disappeared?

e.g. I am pretty sure I replied to you in your protoplanetary disks thread. Also a post of mine in the Haumea thread seems to have disappeared as well. youknowandy's post in the Haumea thread also appears to have reverted to an earlier edit.